Data, available in the literature, of lattice parameter changes in Ni and MgO single crystals neutron-irradiated in reactors are tentatively reproduced using ad hoc ion irradiation experiments. The nature and energy of the ions were selected so that their weighted average recoil spectrum matches at best that of the neutron spectrum inside the reactor. The ion fluence was calculated to obtain similar displacement per atom (dpa) levels. The lattice parameters of the irradiated samples were determined using high-resolution X-ray diffraction. We show that, for Ni, it is not possible to reproduce the neutron data because no lattice parameter change is measured. Main reasons for this failure are the low dpa levels, the small damaged volume and the presence of defects that do not induce significant lattice strain. On the contrary, for MgO which is more prone to experiencing high strain levels upon irradiation, a very close lattice parameter change buildup is obtained between neutron and ion irradiation, providing that the irradiation temperature is significantly increased (by ~473 K).